Determination of the low energy values of 13CH4 transitions in the 2ν3 region near 1.66μm from absorption spectra at 296 and 81K

Abstract

a b s t r a c tThe high resolution absorption spectra of 13 CH 4 were recorded at 81 K by differential absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers and at room temperature by Fourier transform spectroscopy. The investigated spectral region corresponds to the high energy part of the 13 CH 4 tetradecad dominated by the 2m 3 overtone near 5988 cm À1 . Empirical line lists were constructed containing, respectively, 1629 13 CH 4 transitions detected at 81 K (5852-6124 cm À1 ) and 3481 features (including 85 lines of 12 CH 4 ) measured at room temperature (5850-6150 cm À1 ); the smallest measured intensities are about 3 Â 10 À26 and 4 Â 10 À25 cm/molecule at 81 and 296 K, respectively. The lower state energy values were derived for 1196 13 CH 4 transitions from the variation of the line intensities between 81 and 296 K. These transitions represent 99.2% and 84.6% of the total absorbance in the region, at 81 and 296 K, respectively. Over 400 additional weak features were measured at 81 K and could not be matched to lines observed at room temperature. The quality of the resulting empirical low energy values is demonstrated by the excellent agreement with the already-assigned transitions and the clear propensity of the empirical low J values to be close to integers. The two line lists at 81 and at 296 K provided as Supplementary material will enable future theoretical analyses of the upper 13 CH 4 tetradecad.

Information Content Analysis: The Potential for Methane Isotopologue Retrieval from GOSAT-2

“…We note that the 13 CH 4 spectral lines exhibit similar behaviour to the 12 CH 4 spectral lines, but are 15 phase shifted by several nanometres, which is a characteristic of similar rotation-vibrational bands at different transition energies. This spectral region is known as the Tetradecad region and 13 CH 4 absorption is dominated by the 2ν 3 , vibrational band, and 12 CH 4 in this region is characterised by more complex ro-vibrational states, which are described in more detail in Brown et al (2013) and Lyulin et al (2010). It is important to note that Brown et al (2013) issue warnings that significant uncertainties are still attached to the quantum positions of the methane isotopologue lines, especially relating to the effects of 20 atmospheric broadening.…”

“…2 that uncertainty values must be ascribed to the centre position of the isotopologue lines. The methane lines in this region were significantly updated from the previous iteration in HITRAN2008 (Rothman et al, 2009), and were specifically recorded using pure 13 CH 4 and differential absorption spectroscopy (Lyulin et al, 2010). They are judged to be accurate, however, Brown et al (2013) note that "the new HITRAN list of 13 CH 4 above 6170 cm −1 is believed to be incomplete", suggesting that there are additional lines in band 2 that could be leveraged in future HITRAN iterations.…”

“…The spectral lines for 13 CH 4 in band 3 are brand new in the 15 HITRAN2012 database. All spectral lines were captured using FTIR measurements from the Kitt Peak facility and can be ascribed a high degree of confidence (Brown et al, 2013;Lyulin et al, 2010).…”

Response to Review comment 1

“…We note that the 13 CH 4 spectral lines exhibit similar behaviour to the 12 CH 4 spectral lines, but are phase shifted by several nanometres, which is a characteristic of similar rotation-vibrational bands at different transition energies. This spectral region is known as the Tetradecad region and 13 CH 4 absorption is dominated by the 2ν 3 , vibrational band, and 12 CH 4 in this region is characterised by more complex ro-vibrational states, which are described in more detail in Brown et al (2013) and Lyulin et al (2010). It is important to note that Brown et al (2013) issue warnings that significant uncertainties are still attached to the quantum positions of the methane isotopologue lines, especially relating to the effects of atmospheric broadening.…”

“…2, but that uncertainty values must be ascribed to the centre position of the isotopologue lines. The methane lines in this region were significantly updated from the previous iteration in HITRAN2008 (Rothman et al, 2009), and were specifically recorded using pure 13 CH 4 and differential absorption spectroscopy (Lyulin et al, 2010). They are judged to be accurate, however, Brown et al (2013) note that "the new HITRAN list of 13 CH 4 above 6170 cm −1 is believed to be incomplete", suggesting that there are additional lines in band 2 that could be leveraged in future HITRAN iterations.…”